Radio Frequency Identification (RFID) Systems utilize “tags” which are attached to an object to be tracked and have been used in automated pay systems, and the tracking of animals or goods in inventory or in transit. These devices have been around since the 1970's but are burgeoning in the market because of the need for a system which tracks goods which does not need the direct contact that is required for a bar code reader, for example. Currently major retailers are planning on implementing the use of RFID tags on pallets in order to track inventory and plan to start using these on individual items, once the cost of the tags is reduced to about 5 cents per tag.
RFID tags identify themselves to an interrogating radio frequency signal by transmitting back a digitally stored identification number or by generating a random number as the identification number. Considering the large volume of such tags that will be available, the utilization of a limited number of digital bits for storing the identification code could mean that there will be a collision between two devices, each claiming the same code. In this case, the random number generator would be activated to generate different random numbers from the two tags so that they could each be individually identified. Another possibility is the use of the random number generator to generate the unique code directly instead of using a stored identification code.
The major problem in generating a random number is the amount of power that such circuits consume. RFID tags obtain their power by rectifying the received radio frequency interrogation signal, and charging a capacitor to this voltage. Therefore, only an ultra-low power technique can be utilized. Two basic methods are known to generate true random numbers. The first amplifies thermal noise and the second samples a high frequency oscillator with a jittered clock. Both of these methods, however, consume more power than is suitable for an RFID application. A technique known from the “IEEE Transactions on Computers”, April 2003, is the utilization of a high frequency oscillator 102 input to a D-flip flop 106 and using a clock which is a jittered low frequency oscillator 104. If the standard deviation of the jitter is greater than their high frequency oscillator period, then a random bit stream is output. This is illustrated in FIG. 1. The high frequency oscillator used in the published article is a 10-stage 1 GHz ring oscillator which has a large current consumption is therefore unsuitable for use in an RFID tag. Utilizing the prior art technique based upon amplifying thermal noise requires the noise level to be increased to a value larger than the offset voltages due to device mismatches. This, in turn, requires a wide band noise source and a wide band amplifier, both of which consume significant amounts of power.
Accordingly, there is a need for a circuit to produce a true random number bit stream that consumes very small amounts of power.